The present invention relates to an automatic transmission and, more particularly, to an automatic transmission capable of temporarily setting a speed stage other than that which is determined on the basis of the running conditions.
An automatic transmission to be mounted on a vehicle such as an automobile is ordinarily geared at present to change the speed stages stepwise. As is also known in the art, the ordinary automatic transmission mounted on an automobile is constructed to change the transmission passage of a drive force through a gear train by a plurality of frictional engagement means including clutch means and brake means thereby to set one of plural speed stages. In recent years, moreover, the number of speed stages to be set is increased to improve the power performance, to reduce the noises of a high-speed run and/or to improve the fuel consumption. In Japanese Patent Laid-Open No.60-57036, for example, there is proposed a transmission which is enabled to set not only the forward five and one reverse stages but also the so-called "2.5th speed" or "3.5th speed". On the other hand, the shifts of the automatic transmission are ordinarily controlled by using the engine load represented by the degree of throttle opening, the running speed and the engine r.p.m. as control parameters. In case of the stepwise shifts, however, the output torque has to be stepwise changed so that shocks are inevitably generated in accordance with a shift. It has been an important requisite to reduce the noises, for which a variety of trials have been made in the art.
In Japanese Patent Publication No. 61-58697, for example, there has been proposed a system, in which a shift has been stepwise executed by using a timer in case it is determined to jump two steps or more by a control unit on the basis of the throttle opening or the running speed. In Japanese Patent Laid-Open Hei 1-224552, on the other hand, there has been proposed a system, in which a shift is executed by way of an intermediate speed stage having an intermediate one between two adjacent speed stages.
If the number of speed stages to be set is increased, as in an automatic transmission disclosed in Japanese Patent Laid-Open No. 60-57036, the range between the maximum gear ratio and the minimum gear ratio is widened to improve the power performance and the fuel consumption. If, on the other hand, the gear ratios of the individual speed stages are given close values, the torque fluctuations are reduced in case of the stepwise shift to take an advantage for damping the shift shocks. However, the existing vehicular automatic transmission has a restriction upon its gear ratios to be set. If, therefore, the number of gear ratios to be set is increased within the restricted range, two or more speed stages are frequently jumped in the actual run. In this case, it is conceivable to execute the shift via the aforementioned intermediate speed stage. In case of the so-called "jump shift" from the 1st to 5th speeds in the automatic transmission disclosed in Japanese Patent Laid-Open No. 60-57036, the direction of changing the number of revolutions of any rotary member of planetary gear sets composing the gear train is reversed if the shift is executed through their intermediate 2nd, 3rd or 4th speed. As a result, the shift shocks may possibly be deteriorated because the kinetic energy to be absorbed for the period from the start to end of the shift. Depending upon the speed stage selected to pass through, moreover, there occurs the so-called "simultaneous shifts", in which the engagement/release states of three or more frictional engagement means have to be simultaneously switched. In this respect, too, there arises another problem that the shift shocks are deteriorated, or that the shift controls become difficult.
According to the method disclosed in Japanese Patent Publication No. 61-58697, on the other hand, the shift is stepwise set at each intermediate speed stage when the so-called "jump shift" is to be carried out, so that the shift shocks can be reduced. Despite of this advantage, however, the achievement of the target speed stage may be delayed, and the engagement/release the frictional engagement means for executing the shift may have to be frequently switched to invite another disadvantage in the maintenance of durability. In case of the aforementioned shift through an intermediate stage disclosed in the Japanese Patent Laid-Open No. Hei 1-224552 so as to reduce the shift shocks at the time of executing the shift jumping two or more stages, what can be executed by the proposed means is substantially stepwise because an intermediate stage has to be passed in case of a shift between two adjacent stages, thus causing a problem like the aforementioned one.